Speckle patterns typically occur in diffuse reflections of monochromatic light, for example, laser light and is due to interference of many waves having the same frequency, but different phases and amplitudes. The waves add together to provide a resultant wave whose amplitude and intensity vary randomly both in space and time domains. When using laser systems to measure displacement or velocity, the temporal evolution of speckle patterns generated are problematic, particularly, in laser Doppler vibrometry/velocimetry (LDV).
LDV relies on optical coherent detection using laser beams, and, an optical interferometer is a key component to an LDV system. As is known in the field of interferometry, optical beams are used to provide measurements in the form of coherent signals. During the measurement, at least one coherent light beam (measurement signal) is sent to a target to be measured, and, an amount of light (reflected signal) is reflected back to measurement system. The reflected light is combined with at least one reference signal or beam which is coherent with the reflected light to create an interference effect. Due to the frequency difference, the optical intensity of the combined signals changes in the time domain, and these variations can be converted to electrical signals in a single photo-detector (PD) or multiple PDs. Because the frequency of the electrical signal is linearly related to the Doppler shift of the reflected signal, the velocity and the displacement of the target can be derived. LDV systems can be homodyne, where the carrier frequencies of the reference signal or beam and the measurement signal or beam are the same, or heterodyne, where the carrier frequency of the reference signal or beam is different from that of the measurement signal or beam.
Retroreflectors may be attached to a surface of a target to enhance reflections in the direction of the incident beam. These retroreflectors can be micro-prism based reflectors or micro-beads reflectors, and, both of these designs can be used to enhance reflections back to the incoming directions. Whilst a piece of retroreflector with many micro-reflecting-units may be used in order to avoid issues with alignment, such retroreflectors tend to generate speckle patterns.
Whilst it is possible to remove speckle from a reflected beam if the size of the focused laser beam is smaller than the one scattered from the surface of the target or from a reflection unit in case of retroreflectors. However, it is necessary to ensure that the light is just focused on the right location of the target or scatterer, otherwise the reflection can be very weak. Whenever there is an in-plane movement of the retroreflector, the measurement light beam can be shifted from one scatterer to another scatterer with a random height thereby producing an error in the LDV output. The impact of this effect is very similar to that of a speckle pattern.
In the article entitled “Experimental Investigation of the Effect of Speckle Noise on Continuous Scan Laser Doppler Vibrometer Measurements” by Michael W. Sracic and Matthew S. Allen of the University of Wisconsin-Madison, IMAC 2009, a continuous scan laser Doppler vibrometry (CSLDV) system is described in which in which a laser spot is scanned continuously over a structure to make multiple measurements at the same time. The CSLDV measurements are transformed into a set of responses which can be processed using standard identification techniques to extract modes from the measurements. Resampling is used when the scan frequency is high relative to the highest natural frequency of interest, and whilst scanning vibrometers are capable of scanning a relatively high scan frequencies, there is a trade-off between measurement quality and scan frequency due to laser-speckle noise. Scanning in this particular case is needed to map the vibration mode and not for speckle mitigation, and the signals are averaged after demodulation.
However, averaging of the demodulated signals includes errors created by speckle patterns, namely, the signal-to-noise ratio (SNR) of the raw signals is lower due to speckle noise, and, due to the non-linear demodulation process, jumps in the signal output are obtained. As a result, averaging the demodulated signal tends to be difficult.